1. Field of the Invention
The present invention relates to a method for calibrating the amplitude of radiofrequency excitation of an apparatus for imaging by nuclear magnetic resonance (NMR). The invention is primarily applicable to the medical field in which imaging devices of this type are used in diagnostic techniques, in particular for detection of cancers. The invention nevertheless finds potential applications in other fields, especially in the scientific field in which devices based on similar design principles can be employed.
2. Description of the Prior Art
An NMR device essentially includes a magnet for producing a steady and uniform high-strength magnetic field (B.sub.O) or so-called orienting field within an enclosure of interest. A body to be examined is placed within said enclosure and is subjected to a radiofrequency excitation of short duration. As soon as the excitation is discontinued, the magnetic moments of the body particles which have been tilted under the action of this excitation have a tendency to realign themselves with the orienting field. In the course of realignment, said magnetic moments restore the energy collected during excitation in the form of an electromagnetic signal which is detected. Processing of this signal provides information relating to the intimate nature of the body under examination. In methods which make use of imaging stages, NMR machines are also provided with so-called gradient coils which are capable of producing orienting-field supplements, the distribution of which varies according to the coordinates of the points of application. Field gradients applied during the excitation stage as well as or alternatively prior to and/or during reception of the NMR signal accordingly make it possible to code the space. Suitable filtering then makes it possible to extract from the detected signal items of information relating to the nature of the body in selected cross-sections or "slices", images of which can be produced.
The quality of the signal received after excitation is essentially dependent on the efficiency of this excitation. In very general terms, it may be stated that the magnetic moments of the particles are tilted through an angle of 90.degree. in the most favorable cases with respect to the orientation of the orienting field. Said magnetic moments return into alignment with this orientation by precessing. In principle, the means adopted for detecting the precession signal include a coil, the turns of which are located in a plane parallel to the orientation of the field. If the excitation is too weak, the angle of tilt is smaller than 90.degree.. If, on the contrary, the excitation is too strong, the angle of tilt is larger than 90.degree.. In both cases the signal measured at the terminals of the coil is attenuated. The signal-to-noise ratio is less satisfactory in this case. If .theta. denotes the angle of tilt, it may even be positively stated that the amplitude of the detected signal is proportional to sin .theta.. Hence it is an advantage to make .theta. equal to 90.degree..
In actual practice, bodies do not all exhibit the same electromagnetic impedance with respect to the excitation. A fact that can be understood intuitively and has been verified by experience is that bodies of large bulk require a larger amount of energy than small bodies in order to be suitably excited. In the medical field, the medical excitation applicable to a large-bodied patient has to be of considerably higher value than the excitation applicable to a small-bodied patient such as a child, for example. Before beginning all the operations involved in production of the requisite images, it is consequently necessary to carry out an excitation calibration operation. Classically, it is a known practice to subject the body under examination to a series of radiofrequency excitation pulses, the amplitude of which varies from one pulse to the next. The free precession signal (FID) is then measured after each pulse and it is possible to determine the value of excitation at which this signal has passed through a maximum. This excitation value corresponds to the 90.degree. excitation which it is sought to obtain. This method is subject, however, to many drawbacks.
In particular, although acquisition of a free precession signal is particularly simple, it is nevertheless attended by disadvantages. In the first place, this signal is very rapidly evanescent and in addition fails at the moment of measurement to take into account the true value of the signal emitted by the particles by reason of the inhomogeneities of the orienting field. This results in a relative displacement between the true value and the value thus deduced from the 90.degree. excitation. Furthermore, this relative displacement also takes into account the homogeneity of the excitation itself. Finally, the aforementioned calibration entails determination of the 90.degree. pulses. However, the rapid evanescence of the free precession signal in time makes it necessary to measure this signal by means of the so-called spin-echo technique in the case of experimentations followed by production of images. In this technique, the excitation involves a 180.degree. excitation pulse which follows in time the 90.degree. excitation pulse. Regardless of the care devoted to the construction of amplifiers which multiply by two and have the function of converting a 90.degree. pulse to a 180.degree. pulse, the frequency spectrum of these pulses cannot readily be adjusted. It is in fact known that the frequency spectrum of the 180.degree. pulse must be of greater width than the 90.degree. pulse. The 180.degree. pulse is therefore of shorter duration than the 90.degree. pulse (in addition to the fact that its amplitude is double). The correspondence between these different spectra is complex and makes it impossible to perform rapid computation of doubling of the excitation energy.
Experience shows in addition that the influence of the spectral composition of the 180.degree. pulse is predominant in the quality of images formed with such pulses. In particular, faulty calibration of a first 180.degree. pulse results in an artifact in the form of a dotted line at the center of the image. A known expedient for overcoming this difficulty consists in carrying out a double acquisition of the data relating to each image. By combining two acquisitions, this defect can be eliminated but only at the cost of doubling of the time of acquisition of each image. A defect in the calibration of this first 180.degree. pulse also results especially in a strong artifact on the second-echo image in the form of a parasitic image which is symmetrical with the real object about one of tne axes. Similarly, faulty calibration of a second 180.degree. pulse produces a symmetrical ghost image of the object at the time of the third echo. Defects arising from poor calibration of 180.degree. pulses may be removed under certain conditions. To this end, it is possible to produce a modification of the coding by the field gradients at the time of application of these echo pulses. In particular, all the odd-numbered 180.degree. pulses (1, 3, . . .) are applied in the presence of so-called selection gradients, the duration of which is substantially one-half the time interval during which the even-numbered 180.degree. pulses (2, 4, . . .) are applied. This method has the disadvantage, however, of increasing the duration of the selection gradient and therefore of producing a corresponding reduction of the signal measurement time during the sequence considered.